In cellular telecommunications networks, for example according the Long Term Evolution (LTE) standard, the User Equipment (UE) has to send a Buffer Status Report (BSR) to the Evolved Node B (eNB), to which the UE is connected. The BSR contains information about the amount of pending uplink data, i.e., the data available on the Data Link layer in the UE for transmission. Based on the BSRs received from each of the connected UEs, the eNB has an overview of all pending transmission requests and grants uplink data volumes to the UEs.
The standard document 3GPP TS 36.321 (version 10.5.0, release 10) defines in clause 5.4.5 events that require the Medium Access Control (MAC) sublayer of the Data Link layer to send a BSR. The standard document further defines different sizes for the BSR depending on the number of channels for which uplink data is pending in the UE.
When the UE has received a grant for uplink data transmission and when an event requires the transmission of a BSR, the MAC sublayer has to reserve an amount of bytes corresponding to the size of the BSR, in order to prevent that the reserved bytes are used for user data transmission. The Data Link layer then performs an uplink data processing of the pending uplink data.
As long as the number of channels with pending uplink data does not change during the uplink data processing, the number of bytes actually needed for the BSR does not change and is consistent with the number of reserved bytes. If, however, the number of channels, for which uplink data is pending, is being reduced during the uplink data processing, fewer bytes may actually be needed for the BSR. As a consequence, some of the bytes reserved in the granted uplink data volume are not used for uplink data transmission. It is essential for the performance of the UE to efficiently use all granted uplink resources. In the sketched situation, however, uplink resources may be wasted.
In order to fully use the granted data volume, the reservation could be corrected and the uplink data processing could be repeated based on the corrected reservation. However, such a repetition of grant reserving and uplink data processing delays the transmission of the pending data and additionally requires computational and power resources. Even worse, the size of the BSR can change again as a result of the repeated uplink data processing based on the corrected reservation, which leads to a resource-consuming iteration of the uplink data processing.
An alternative approach could be to increase the size of the last Radio Link Control (RLC) Packet Data Unit (PDU). Such a post-processing can become quite complex, especially in the case of modifying an already generated RLC Status PDU or retransmitting an RLC User PDU. The complex post-processing would not only require additional processing time, but also additional memory resources.